1. Field of the Invention
The present invention relates in general to a piezoetectric/electrostrictive actuator, and more particularly to a uni-morph, bi-morph or other types of piezoelectric/electrostrictive actuators or actuator units which undergo displacement in the form of bending, deflection or flexure and which can be used for ink jet print heads, microphones, sounding bodies (such as loudspeakers), various resonators and vibrators, and other components or devices.
2. Discussion of the Related Art
As a means for raising a pressure in a pressure chamber formed within a substrate of an actuator unit, there is recently known a piezoelectric/electrostrictive element formed on a wall partially defining the pressure chamber, for changing a volume of the pressure chamber due to displacement of the piezoelectric/electrostrictive element. Such a piezoelectric or electrostrictive actuator unit may be used for each of a plurality of ink pumps of a print head used in an ink jet printer, for example. To effect printing by the print head, an ink material is supplied to the pressure chamber to fill the same, and the level of ink pressure within the pressure chamber is raised due to displacement of the piezoelectric/electrostrictive element, so that a droplet of the ink is jetted through a nozzle which communicates with the pressure chamber.
Referring to FIGS. 5 and 6, there is illustrated one of the actuator units used in an ink jet print head, wherein the reference numeral 25 denotes an actuator portion which includes a substrate 24 consisting of a closure plate 18 and a spacer plate 20 both made of a metal, synthetic resin or ceramic material, and a plurality of piezoelectric/electrostrictive elements 28 formed on an outer surface of the closure plate 18 of the substrate 24. The spacer plate 20 has an array of windows 22 (three windows in this example) formed through its thickness. The spacer plate 20 and the closure plate 18 are superposed on each other and formed integrally into the substrate 24, so that one of opposite openings of each window 22 is closed by the closure plate 18, thereby to provide, within the substrate 24, a plurality of pressure chambers 26 which correspond to the respective windows 22. The piezoelectric/electrostrictive elements 28 are secured to the outer surface of the closure plate 18 of the substrate 24 such that the piezoelectric/electrostrictive elements 28 are aligned with the respective pressure chambers 26. In operation, the piezoelectric/electrostrictive elements 28 are actuated to cause deformation and resulting volumetric change of the pressure chambers 26 for changing the pressure within the pressure chambers 26.
Each actuator unit of the ink jet print head further has an ink nozzle member 16, as shown in FIGS. 5 and 6, which consists of a metallic nozzle plate 4 having a plurality of nozzles 2, a metallic orifice plate 8 having a plurality of orifices 6, and a channel plate 10, which are superposed on each other such that the channel plate 10 is interposed between the plates 4, 8. These plates 4, 8, 10 are bonded together into the ink nozzle member 16, so as to form ink discharge holes 12 and an ink supply channel 14. The plurality of nozzles 2 communicate with the respective pressure chambers 26 of the actuator portion 25 through the respective ink discharge holes 12 formed within the ink nozzle member 16, while the plurality of orifices 6 communicate with the respective pressure chambers 26 of the actuator 25, and also with the ink supply channel 14 formed within the ink nozzle member 16.
The ink nozzle member 16 and the actuator portion 25 are superposed on each other, and bonded together into an integral structure of the actuator unit. Generally, an ink jet print head uses many actuator units arranged side by side. In such ink jet print head, the actuator portion 25 of each actuator unit used as an ink pump is adapted to deform the pressure chambers 26 for changing the pressure therein, based on the actuation of the piezoelectric/electrostrictive elements 28, so that droplets of the ink material in the pressure chambers 26 are jetted or discharged through the appropriate nozzles 2 to effect printing. As an attempt to provide an ink jet print head which is capable of performing highly accurate and high-quality printing, a modified form of the actuator unit is proposed, wherein a relatively large number of pressure chambers 26 are formed within the substrate 24 of the actuator portion 25 at a relatively small pitch, and a correspondingly large number of nozzles 2 are formed through the nozzle plate 4.
The actuator portion 25 of each actuator unit constructed as described above, however, has suffered from a problem in terms of the operating characteristics of the piezoelectric/electrostrictive elements 28. That is, among the plurality of piezoelectric/electrostrictive elements 28 which are formed in a row on the substrate 24 so as to correspond to the respective pressure chambers 26, two outermost elements 28 which are located at the opposite ends of the row are inferior in the operating characteristics to the other or two inner elements 28 interposed therebetween. More specifically described, the amount of flexural deformation of each piezoelectric/electrostrictive element 28 of the actuator portion 25 constructed as described above was measured by applying a suitable voltage to each of the elements 28. The result of the measurement shows that the amounts of flexural deformation of the above-indicated two outermost elements 28 are considerably small, that is, the amounts of flexural deformation of the two outermost elements 28 are smaller by about 30 per cent as compared to those of the inner elements 28 interposed between the two elements. A variation or fluctuation in the amount of flexural deformation or inconsistent operating characteristics of the piezoelectric/electrostrictive elements 28 of the actuator portion 25 undesirably causes inconsistency or fluctuation in the ink-jetting capability of the print head and deterioration of the quality of printed images produced by the print head, when the above-constructed actuator units are used as the ink pumps for the ink jet print head.